1. Field of the Invention
The present invention relates to a light transmitting medium in an infrared wavelength band and, in particular, to a hollow waveguide suitable for optical energy transmission of high power in the infrared wavelength band and a method of manufacturing the hollow waveguide.
Also, the present invention relates to a light transmitting medium in an ultraviolet wavelength band and, in particular, to a hollow waveguide suitable for optical energy transmission of high power in the ultraviolet wavelength band and a method of manufacturing the hollow waveguide.
2. Description of the Related Art
(1) Optical Waveguide for Transmitting Infrared Light
Infrared light having a wavelength of 2 μm or more is utilized in various fields such as medicine, industrial working, measurement, analysis, and chemistry. In particular, each of an Er-YAG laser having a wavelength band of 2.94 μm, a CO laser having a wavelength band of 5 μm, and a CO2 laser having a wavelength band of 10.6 μm has high excitation efficiency and can develop high power and has large absorption also for water, and hence is utilized as a light source for medical laser equipment and for industrial processing.
A conventional silica-based optical fiber used for communications produces a large loss of transmission for laser light having a wavelength of 2 μm or more because infrared absorption caused by molecular vibration is large. For this reason, the usual silica-based optical fiber cannot be used as a waveguide for transmitting this infrared laser light. Hence, there have been actively made developments of a new type optical waveguide which can be applied even to an infrared wavelength band of wide application area.
A hollow waveguide provided with a dielectric layer transparent in the wavelength band of light to be transmitted has been developed as an optical waveguide for transmitting infrared light having a wavelength of 2 μm or more and has been proved to have excellent transmission characteristics.
FIG. 5 is a sectional view showing a conventional hollow waveguide 4. The hollow waveguide 4 is constructed of a glass capillary 41, a metal layer 42 formed on the inside wall of the glass capillary 41, a dielectric layer 43 formed on the metal layer 42, and a hollow region 44 formed as a core inside the dielectric layer 43. The glass capillary 41 is a base material for holding the mechanical strength of the hollow waveguide 4. The dielectric layer 43 is transparent in the wavelength band of light to be propagated and usually has a thickness of submicron or less and has a thickness set at an optimum value according to the wavelength of light to be propagated. The metal layer 42 has large optical absorption in the wavelength band of light propagating through the hollow waveguide 4, and hence optical energy does not penetrate deeply into the metal layer 42. Therefore, it is essential only that the thickness of the metal layer 42 in contact with the dielectric layer 43 is a skin depth or more. Light propagating through the hollow waveguide 4 is repeatedly reflected by the boundary between the hollow region 44 and the dielectric layer 43 and the boundary between the dielectric layer 43 and the metal layer 42, thereby being propagated.
Specifically, there is disclosed a hollow waveguide having the metal layer 42 made of silver on the inside wall of the glass capillary 41 by plating and the dielectric layer 43 formed on the metal layer 42 by thermosetting a solution of the precursor of polyimide or a solution having an olefin polymer dissolved therein (for example, Japanese Patent Application Laid-Open Nos. 8-234026 and 2002-71973).
Even if the metal layer 42 is formed on the glass capillary 41 having an extremely smooth surface by plating, the surface roughness of the metal layer 42 becomes larger as the thickness of the metal layer 42 becomes larger. The metal layer 42 having a thickness of several hundred angstroms functions optically to a sufficient degree. Hence, the metal layer 42 is formed in as thin a thickness as possible so as to prevent its surface from losing a mirror-smooth state.
An organic material constructing the dielectric layer 43 such as polyimide and olefin polymer has an infrared absorption peak wavelength specific to the material. However, because the dielectric layer 43 is formed in a sufficiently thin thickness, propagating light in an infrared range except for this infrared absorption peak wavelength is hardly attenuated in the dielectric layer 43. For this reason, the dielectric layer 43 can be assumed to be a transparent material through which the propagating light reaches the metal layer 42. In particular, specific organic materials such as polyimide and olefin polymer do not have a large infrared absorption peak in the oscillation wavelength band of the Er-YAG laser, the CO laser, and the CO2 laser, so that the hollow waveguide 4 can transmit practically important infrared laser light with low transmission loss. Moreover, since the metal layer 42 is formed in a very thin thickness by plating to keep the inside wall of the hollow waveguide smooth, the hollow waveguide 4 can transmit not only infrared laser light but also visible light as guide light.
In addition to the hollow waveguide 4 having the dielectric layer 43 made of the organic material as described above, a hollow waveguide having a dielectric layer formed by chemically altering a part of a metal layer is also developed. For example, there has been known a hollow waveguide in which a silver iodide layer produced by iodinating a part of a metal layer, which is made of silver on the inside wall of a glass capillary by plating, functions as a transparent dielectric layer. The silver iodide is an inorganic substance which is transparent in the infrared wavelength band and does not have an infrared absorption peak specific to the substance like a polymer substance. Thus, the silver iodide can transmit light in the infrared wavelength range with low transmission loss.
In addition to the above-described glass capillary, a resin tube made of fluorine resin or the like having excellent flexibility is proposed as a base material for providing the hollow waveguide with the mechanical strength. Further, for a laser probe mounted at the tip of a long optical transmission line and uses not specially requiring flexibility, there is proposed a base material made by polishing the inside surface of a stainless pipe or the like which is mechanically stronger than the glass capillary. Still further, there is proposed a hollow waveguide in which a pipe itself made of precious metal such as silver is used as a base material so as to save a step of forming a silver layer by plating and has its inside surface polished to a mirror-smooth state to form a dielectric layer.
(2) Optical Waveguide for Transmitting Ultraviolet Light
On the other hand, ultraviolet light having a wavelength of 250 nm or less is utilized in various fields such as medicine, industrial processing, measurement, analysis, and chemistry. In particular, a KrF laser having a wavelength of 248 nm, an ArF laser having a wavelength band of 193 nm, an excimer laser such as an F2 laser having a wavelength band of 157 nm, or a Q switch YAG harmonic laser can produce high power and is important as a light source or semiconductor photolithography equipment, fluorescence analysis, medical equipment, and industrial processing.
A conventional silica-based optical fiber used for communications can transmit light having a wavelength of approximately 200 nm or more with low transmission loss. Moreover, recently, a solid-type silica-based optical fiber has been improved particularly for the purpose of transmitting ultraviolet light.
Absorption in an ultraviolet band is an absorption band caused by electron transition and absorption spectrum characteristic is substantially affected by impurities and structural defects contained in the silica-based optical fiber. Silica-based optical fibers widely used at present have been purified to such an extent that absorption by metallic impurities can be neglected. Hence, transmittance in the ultraviolet band of the silica-based optical fiber is determined by structural defects in silica glass that depend on the manufacturing conditions.
Fine structural defects depend on the manufacturing conditions: for example, oxygen depletion type defects and oxygen excess type defects are caused by an oxidizing/reducing atmosphere when the fiber is manufactured. In a method of manufacturing an optical fiber in which soot-like silica particles are dehydrated in a halogen atmosphere and then are heat-treated to be altered to transparent glass, oxygen depletion type defects are caused to decrease transmittance in the ultraviolet band. Because the content of an OH group is varied by the dehydration treatment, the transmittance in the ultraviolet band depends on the OH group.
In silica glass anhydride subjected to the dehydration treatment, an absorption band caused by Si—Si oxygen depletion type defects is observed at wavelengths of 245 nm and 163 nm. Further, in silica glass made by sintering soot in a reducing atmosphere, an absorption band is observed at a wavelength of 240 nm.
In contrast to this, silica glass made by sintering soot in a He gas atmosphere contains a high concentration of the OH group and does not have an outstanding absorption band observed in a wavelength range from 200 nm to 400 nm. As described above, the transmittance of the silica-based optical fiber for the purpose of transmitting ultraviolet light depends on the content of the OH group.
On the other hand, apart from the solid-type silica-based optical fiber like this, there is proposed, for the purpose of transmitting ultraviolet light, a hollow waveguide made by depositing aluminum on the inside of a hollow glass capillary by a metal organic chemical vapor deposition method (MOCVD) (Optical Alliance, July, 1999, pp. 20-22). The advantage of the hollow waveguide can endure higher energy density than the silica-based optical fiber. The maximum transmission energy density of the solid-type silica-based optical fiber is approximately 50 mJ/cm2, whereas the hollow waveguide can transmit a beam having a transmission energy density of 2 J/cm2 or more. Further, even in the case of the silica-based optical fiber improved for the ultraviolet light, the wavelength of 193 nm of the ArF laser is the shortest limit and it is difficult to transmit light in the vacuum ultraviolet band of shorter wavelength. In contrast to this, the hollow waveguide having an aluminum thin film deposited thereon can transmit light having as short a wavelength as approximately 130 nm and can transmit also the F2 laser having a wavelength of 157 nm.
(1) Problems of Hollow Waveguide for Transmitting Infrared Light
However, the conventional hollow waveguide for transmitting infrared light has the following problems. That is, the hollow waveguide 4 using the glass capillary 41 has flexibility but has a possibility of being suddenly broken when it is held in a small bending radius for a long time. Further, the hollow waveguide 4 has a possibility of being broken when it is inserted into the human body or used in use environment where impact or external force is applied thereto, which is not so desirable.
A hollow waveguide using a resin tube made of fluorine resin or the like as a base material has a lower possibility of being broken than a hollow waveguide using a glass capillary but is irregularly varied in a sectional shape and in the bending shape of the whole transmission line by impact or external force, thereby being easily varied in transmission characteristics. Further, in the resin tube, as compared with the glass capillary, the surface of the inside wall is rough and is hard to improve by polishing or etching to such an extent that is achieved in the glass capillary. For this reason, transmission loss is increased in the case of transmitting light having short wavelengths such as visible light.
Moreover, the entire loss of light propagating in the hollow waveguide is converted to heat, and hence the hollow waveguide using a glass capillary or a resin tube having small thermal conductivity as a base material might cause local heating.
In the hollow waveguide formed of the glass capillary, the silver layer formed on the inside wall of the glass capillary, and the dielectric layer made of silver iodide formed by iodinating the inside wall of the silver layer, the silver layer needs to be formed in a thickness larger than an optically contributable thickness so as to avoid the silver layer to be iodinated from being lost. As a result, the surface of the inside wall of the hollow waveguide having silver iodide applied thereto is degraded in the roughness to damage the mirror-smooth state of the silver layer, which is disadvantageous specially for the transmission of light having short wavelengths such as visible light.
When a hollow waveguide is used for use not requiring flexibility, a metal pipe can be advantageously used as the base material of the hollow waveguide in that the metal pipe has large mechanical strength and high thermal conductivity. However, a conventional hollow waveguide, which uses a stainless pipe having its inside surface polished to a mirror-smooth state as a base material and has a silver layer formed on the inside wall of the stainless pipe by plating, loses the smoothness of the surface of the inside wall by plating and hence is remarkably inferior in smoothness to the hollow waveguide formed by plating the glass capillary with silver.
FIG. 6 shows a wavelength-loss characteristic when white light is propagated through a metal hollow waveguide, which is formed of a stainless pipe hating its inside wall polished to a mirror-smooth state and a silver layer formed by plating the inside wall of the stainless pipe with silver. FIG. 7 shows a wavelength-loss characteristic when white light is propagated through a metal hollow waveguide, which is formed of a glass capillary and a silver layer formed by plating the inside wall of the glass capillary with silver. Each of the metal hollow waveguides has a length of 40 cm and an inside diameter of 0.7 mm and the same thickness of the silver layer formed by plating.
As shown in FIGS. 6 and 7, the loss of the metal hollow waveguide formed of the stainless pipe is considerably larger than the loss of the metal hollow waveguide formed of the glass capillary. In particular, as the wavelength becomes shorter, the loss becomes larger. It is thought that this is because the inside surface of the stainless pipe is not polished to the same degree of smoothness equal as in the case of the glass capillary or that this is because even if the stainless pipe and the glass capillary are equal to each other in the degree of smoothness, the surface roughness of the silver layer is different between the stainless pipe and the glass capillary because of difference in the base material, that is, the smoothness of the base material cannot be held on the surface of the silver layer. With this characteristic, the hollow waveguide formed of the stainless pipe having its inside surface polished to a mirror-smooth state and the silver layer formed by plating the polished inside surface of the pipe with silver is inferior in transmission loss to the hollow waveguide formed of the glass capillary and the silver layer formed by plating the glass capillary with silver.
Further, the hollow waveguide formed of the glass capillary has problems of being lower in resistance to external force, easily causing local heating because of using glass having low thermal conductivity as the base material, and silver plating being easily peeled off.
Still further, a hollow waveguide is also studied in which in place of the hollow waveguide including a stainless pipe having its inside surface polished and a silver layer formed by plating the polished inside surface with silver, a silver pipe itself has its inside surface polished to eliminate a step of plating the inside surface with metal to keep the surface roughness of the inside wall of the pipe. In this hollow waveguide, the whole base material is silver and hence cost is very much increased. It is known that not only silver but also gold and copper are suitable as such a metal material of a hollow waveguide that is optically contributable to transmitting a laser light in an infrared wavelength band with low loss. It is difficult in practical use in terms of cost to form the base material of the hollow waveguide of gold. Further, silver and copper are remarkably discolored by oxidation or sulfuration, and therefore, it is not preferable that the base material of the hollow waveguide is made of these materials and exposed to outside environment. Still further, the hollow waveguide using any one of these materials as the base material is easily plastically deformed even by small bending and hence is remarkably degraded in transmission characteristics particularly in use environment where the hollow waveguide is repeatedly bent.
(2) Problems of Hollow Waveguide for Transmitting Ultraviolet Light
A conventional hollow waveguide for transmitting ultraviolet light has the following problems.
That is, when a pulse of ultraviolet light is entered into a silica optical fiber commonly used, even if an initial transmittance is excellent, the transmission characteristic is degraded as time elapses during the irradiation of light (Appl. Opt. 27, 1988, p. 3124).
As described above, there have been developed also silica optical fibers for transmitting ultraviolet light that can transmit even ultraviolet light with stability by adjusting the concentration of the OH group. However, such silica optical fibers yet have problems in transmitting the ArF laser and the KrF laser having wide application fields. Further, the silica optical fibers can not stably transmit the F2 laser having a short wavelength and a high-power pulse laser for a long time.
On the other hand, an aluminum hollow waveguide is more promising in the transmission of ultraviolet laser of 190 nm or less in wavelength or of high power intensity than the silica optical fiber. However, in the above-described hollow waveguide having an aluminum thin film deposited inside the silica glass capillary by the MOCVD method, the aluminum thin film does not have a sufficiently strong adhesion force and hence is easily peeled off. In particular, in the case of transmitting the ultraviolet light by the hollow waveguide, the hollow space is commonly evacuated to a vacuum or filled with rare gas so as to prevent oxygen in the air from being altered to ozone absorbing the ultraviolet light to increase transmission loss. For this reason, there is a possibility that when the aluminum thin film deposited inside does not have a sufficiently strong adhesion force, the aluminum thin film might be peeled off when the gas is sucked or introduced.
Further, to deposit the aluminum thin film inside the silica glass capillary, an expensive MOCVD apparatus is required.
Still further, the hollow waveguide using the glass capillary like this is low in resistance to the external force and hence might be broken by impact or bending.